1. Field of the Invention
This invention relates to a developing device for use in an image forming apparatus such as copier and printer to supply a developing agent to a photoreceptor so as to develop a latent image into a toner image, and more particularly to a developing device which uses a two-component developing agent consisting of toner and carrier.
2. Description of the Related Art
Heretofore, there has been known a developing device for use in an image forming apparatus to develop an electrostatic latent image formed on the surface of a photosensitive drum into a toner image with use of a two-component developing agent consisting of toner and carrier.
Such a developing device is provided with a housing for accommodating a developing agent, developing agent supplier means for supplying the developing agent in the housing to the surface of the photosensitive drum to develop a latent image into a toner image, developing agent mixing means for mixing the developing agent in the housing while at the same time feeding the developing agent in a certain direction, and toner replenishing means for replenishing toner into the housing.
The developing agent supplier means supplies toner on a certain area (developing area) of the drum surface to develop the latent image into a toner image within the area. As the toner in the housing is used up in accordance with progress of the image formation, the toner replenishing means replenishes toner into the housing. The developing agent mixing means agitates the developing agent, namely mixes the toner particles with the carriers which are electrically charged to convey the toner particles together with the charged carriers owing to electrostatic friction.
A known technology relating to agitation and transport of developing agent is disclosed in, for example, Japanese Unexamined Patent Publication No. 10-142942. The publication discloses a four-membered tandem type developing device for use in a color copier comprising: developing agent supplier means for supplying a developing agent onto the surface of a photosensitive drum; first mixing/transporting means disposed near the developing agent supplier means to agitate the developing agent while transporting the developing agent in a first direction thereof; second mixing/transporting means disposed underneath the first mixing/transporting means to agitate the developing agent while transporting the developing agent in a second direction opposite to the first direction; and feeder means coaxially provided along with the longitudinal direction of the developing agent supplier means to feed the developing agent transported by the second mixing/transporting means toward the first mixing/transporting means in response to driving of the developing agent supplier means.
The above-mentioned conventional developing device has room for improvement in the aspect of agitation and transport of developing agent, prevention of undesired stay of developing agent, and reducing the dimensions of the developing device itself. Particularly, as for the dimensions of the device itself, since the feeder means is provided coaxially with the developing agent supplier means (magnet roller) to feed the developing agent from the second mixing means (lower mixing means) to the first mixing means (upper mixing means), the device itself is inevitably large in the axial direction of the magnet roller by a dimension corresponding to the axial length of the feeder means which axially protrudes beyond the developing area.
In view of the above problems residing in the prior art, it is an object of the present invention to provide a developing device of a compact size particularly in the axial length thereof that enables to ensure sufficient agitation of a developing agent and prevention of undesired stay of the developing agent.
In one aspect of this invention, a developing device for use in an image forming apparatus comprises: a developing agent supply unit which supplies a developing agent to a photoreceptor; a first mixing unit provided near the developing agent supply unit to convey the developing agent in a first conveyance direction while mixing the developing agent; a second mixing unit provided underneath the first mixing unit to receive the developing agent from the first mixing unit at a downstream end along the first conveyance direction so as to supply the developing agent to the developing agent supply unit while mixing and conveying the developing agent in a second conveyance direction; and a developing agent scoop-up unit provided substantially in the middle in a height direction between the first mixing unit and the second mixing unit and at a downstream end of the second mixing unit along the second conveyance direction to receive the developing agent from the second mixing unit and draw up the received developing agent to the first mixing unit.
With this arrangement, the developing agent is sufficiently mixed while conveyed in the first and second conveyance directions by the first and second mixing units. Further, this arrangement enables to provide an image forming apparatus of a compact size, particularly in the axial direction of the developing device while preventing stagnation of the developing agent.
These and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawings.